The development of coagulation factor replacement therapy has transformed the lives of many individuals with hemophilia. Hemophilia is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation. Patients with hemophilia do not produce adequate amounts of Factor VIII or Factor IX proteins, which are necessary for effective blood clotting. In severe hemophiliacs even a minor injury can result in blood loss that continues for days or weeks, and complete healing may not occur, leading to the potential for debilitating permanent damage to joints and other organs, and premature death.
One type of hemophilia, Hemophilia B, is an X-linked bleeding disorder caused by a mutation in the Factor IX (FIX) gene, resulting in a deficiency of the procoagulant activity of FIX. Hemophilia B patients have spontaneous soft tissue hemorrhages and recurrent hemarthroses that often lead to a crippling arthopathy. Current treatment for these patients includes an intravenous administration of recombinant FIX. However issues of cost and relatively rapid clearance of FIX from the circulation make developing a long-acting FIX a challenging task.
Commercial availability of FVIII and FIX has lead to improved control of life-threatening bleedings episodes. Many patients receive prophylactic therapy, which reduces the risk of bleeding and its associated complications. However, a significant proportion of patients (10-30%) develop inhibitory antibodies to exogenously administered FVIII and FIX. Administration of FVIIa, which is a bypassing product, can induce homeostasis and provide an effective treatment for patients with inhibitory Abs.
Recombinant FVIIa (NovoSeven®) is commercially available and was approved in 1996 for treatment of bleeding episodes in hemophilia patients with inhibitors. However, rFVIIa is rapidly cleared with a terminal half-life of 2.5 hours. As a result, patients generally require multiple, frequent infusions (2-3 doses given in 2-3 hour intervals) to achieve adequate homeostasis following a mild to moderate bleed. Consequently, there is much interest in developing a long-acting form of FVIIa that would prolong the duration of haemostatic activity following a single dose and allow much less frequent dosing. A long-acting FVIIa would also increase the feasibility of long-term prophylactic therapy.
Various technologies are being developed for prolonging the half-life of FVIIa. However, the challenge is to achieve a prolonged half-life of this protein while preserving its biological activity and ensuring that the modifications do not induce significant immunogenicity.